Monday, April 30, 2012

When I give brewery tours at Ale Asylum, a homebrewer or two can usually be counted among the attendees. Aside from the occasional showoffs who ask a bunch of questions to which they already know the answers, I very much enjoy talking with other brewers. I feel the same way about people with biology and chemistry backgrounds who may not be familiar with the breadth of the brewing process, but understand certain aspects of it very deeply. On a recent tour, a homebrewer asked me a number of questions that demonstrated both a strong grasp of brewing science and a desire to learn more. Given his level of knowledge, one of his questions surprised me: how much yeast he should pitch?

In my opinion, knowing how much yeast to pitch is one of the fundamentals of brewing consistently excellent beer. It's one of the first concepts that new brewers should learn, but it usually isn't. Anyway, the ideal amount of yeast to pitch will depend on the type of beer you're brewing. For most ales, I like to shoot for 0.75 million viable cells per milliliter of wort in the fermentation tank per degree Plato of original gravity. For lagers, as well as beers like Alts and Kolsches that are fermented by ale yeasts at cool temperatures, I like to target 1.25 million viable cells/mL/P. To convert from specific gravity to degrees Plato, you can use the following equation:

P = ((116.716 x SG - 569.851) x SG + 1048.046) x SG - 594.914.

If you plan to ferment 5.5 gallons of 15P ale, your target pitch should be 0.75 x [5.5 gal x 3785.4 mL/gal] x 15 / 1000 = 234 billion cells. If you're going to use dry yeast, you can assume each gram contains 14 billion viable cells (20 billion cells at 70% viability). I got the number from this BYO article, and it's worked very well for me. Continuing with the same example, 234 billion cells translates to 234/14 = 17 grams of dry yeast.

If you're going to use liquid yeast, you can assume Wyeast Activator packs and White Labs vials each contain 100 billion viable cells minus 5 billion cells for each week after the manufacturing date (Wyeast prints manufacturing dates on its smack packs, and the Best Before date on each White Labs vial is four months after its manufacturing date). With liquid yeast, it's far more economical to make yeast starters than pitch multiple packages. The Mr. Malty and YeastCalc websites will calculate yeast starter volumes for you, but they each have limitations that make them somewhat cumbersome: the Mr. Malty program only calculates the volumes of one-step starters and the number of packs/vials needed to pitch into them, while YeastCalc makes you guess your starter volumes to arrive at your final cell counts by trial and error. My pitching spreadsheet (available at http://sites.google.com/site/republicbrewpub/ as Yeast_Gallons.xlsx) is probably less intuitive, but it'll use your starting cell count to calculate the ideal number of starters and the required volume of each. If you didn't read it the first time around, this post will help you compensate for the impact of a starter on the gravity and hop bitterness of your main batch.